1. Field of the Invention
The present invention relates to a method of fabricating an optical fiber preform by hydrolyzing a glass raw material gas in flame to generate glass particles, and depositing the particles on a rotating starting material.
2. Description of the Related Art
Up until now, various methods have been proposed for manufacturing optical fiber preforms. Among these methods is an Outside Vapor Phase Deposition Method (OVD method). In this method, glass particles generated by a burner flame are deposited and adhered on a rotating starting material while relatively reciprocating the burner or the starting material to synthesize a porous preform. The preform is desiccated and sintered in an electric furnace. The foregoing method has been widely used because the method can provide an optical fiber perform having a relatively arbitrary refractive index distribution and can mass-produce preforms having a large diameter.
FIG. 1 shows an outline of one example of an apparatus for fabricating an optical fiber preform. In the drawing, a starting material, on which glass particles (soot) are deposited, is constituted by welding a dummy rod 2 at both ends of a core rod 1, and both ends are supported by an ingot chuck mechanism 4 so as to be rotatable about an axis thereof. A freely movable burner 3 is arranged in the direction of the starting material. Vapor of an optical fiber source material such as SiCl4 and combustion gas (hydrogen gas and oxygen gas) are blown from the movable burner to the starting material, and soot generated by the hydrolysis in an oxyhydrogen flame is deposited on the starting material to form a porous optical fiber preform. Reference numeral 5 represents an exhaust hood.
The burner 3 is supported by a burner guide mechanism (not shown) so as to be reciprocal in a longitudinal direction of the starting material and retractable from the starting material, and ejects a flame towards the starting material rotating about the axis. By depositing glass particles generated by the hydrolysis of the source material gas in the flame, a porous preform is fabricated. Next, by passing the porous preform through a heater portion of a heating furnace (not shown), it is desiccated and vitrified to become an optical fiber preform.
For synthesizing glass particles and depositing soot (glass particles) on a starting material, a concentric multiple tube burner has been conventionally used. However, the burner having such structure can not sufficiently mix a glass raw material gas, a flammable or burnable gas and a combustion assisting gas to generate a sufficient amount of glass particles. As a result, the manufacturing yield has not been improved and it has been difficult to achieve a high-speed synthesis.
In order to solve the problem, Japanese Patent No. 1773359 has proposed a multi-nozzle type burner, in which a plurality of small diameter combustion assisting gas-ejecting ports (hereinafter, referred to as small diameter ejecting ports) are arranged in a burnable gas-ejecting port so as to surround the central source material gas-ejecting port.
Further, Japanese Patent No. 3543537 has proposed a method of preventing the disturbance of a source material gas flow, in which, when denoting the focal length of a plurality of small diameter ejecting ports by L1 and the distance from the tip of the small diameter ejecting port to the glass particle deposition plane of a preform by L2, it is proposed that L1 is larger than L2. Inversely, Japanese Patent Application Laying-Open No. 2003-226544 has disclosed that the deposition efficiency can be improved by making L1 smaller than L2 to enhance the mixing efficiency of the gas.
However, in the OVD method, in which glass particles generated in a burner flame are deposited and adhered onto a rotating starting material while causing the burner or the starting material to relatively reciprocate, the weight of the preform increases and the diameter of a deposited body increases as the deposition progresses. Accordingly, as the deposition progress, the gas amount is usually increased to adjust the density of the deposited body. The deposition is continued until a starting material having an initial diameter of 50 mm attains a diameter of 300 mm.
During an early stage of deposition, since the deposition area is small, the deposition is performed at a small linear velocity of gas with a small amount of gas supplied. Consequently, the flow of glass particles is easily disturbed by gas ejected from the small diameter ejecting ports, and the disturbance decreases the deposition efficiency. In the second half of deposition, since the deposited body has an increased diameter and the deposition area has increased, the deposition is performed at a large linear velocity of gas with an increased amount of gas supplied. As a result, although the disturbance of the flow of glass particles by gas ejected from the small diameter ejecting ports is small, there is a problem that a large linear velocity of gas reduces the mixing ratio of gases and thus the deposition efficiency is decreased.